Vehicle brake systems equipped with an electric servomotor are generally known, as in document French Patent No. 0311580, for example. Such a servomotor is equipped with a reaction disc whose sensitivity is difficult to adjust; in particular, the sensitivity of the reaction disc depends on the external temperature. Another disadvantage is the cost of the reaction disc.
An object of the present invention is a hydraulic-reaction brake system equipped with an electric servomotor that is independent of temperature, whose fabrication is less complex than known fabrications, and whose reliability and endurance is greater than one equipped with a reaction disc.
To that end, the object of the present invention is a vehicle brake system with an electric servomotor of the type described above, characterized in that                the primary piston delimiting the primary chamber in the master cylinder is formed from a body        that, on the exterior of the master cylinder, is extended by a cylindrical skirt forming an external chamber housing a reaction spring one end of which presses against a retaining organ integral with the skirt;        that has an axial through hole connecting the rear of the external chamber and the primary chamber;        the actuator piston cooperates directly with the skirt to push the primary piston;        there is a hydraulic reaction piston;        housed in the bore hole of the body of the primary piston;        integrally bearing an axial thrust rod on the exterior;        equipped with a stop against which the other extremity of the reaction spring rests;        which said stop rests against the rear of the external chamber to push the primary piston or to be pushed by it.        
The brake system according to the invention as described above has the advantage of especially simple fabrication and operation independent of the outside temperature with respect to pedal feel. This solution is especially reliable because it does not include a reaction disc, which is generally made of a rubbery material.
According to a particularly advantageous characteristic, the reaction spring consists of a principal spring and an auxiliary spring combined in series, separated from one another by a floating support that is mobile with respect to a primary piston stop to limit the expansion stroke of the principal spring, the auxiliary spring being of reduced hardness with respect to that of the principal spring so it can be compressed before the principal spring. The reaction spring is thus composed of an auxiliary spring with the interposition of a floating support to compensate for the drag of the brake against the brake disc, wherein this does not affect pedal feel.
According to another advantageous characteristic, the retaining organ is a disc traversed by the thrust rod, which is of particular interest if, according to another advantageous characteristic, the thrust rod has a shoulder that cooperates with the retaining organ to define an extreme end-of-travel position for the reaction piston.
According to another advantageous characteristic, the stop has a collar that cooperates with the floating support in compressing the auxiliary spring to limit the compression of the auxiliary spring and thereby press directly upon the floating support.